FlightGear doesn't necessarily have a real "fictional side" - it's just that we all needed to get started somehow. Contributing a completely new aircraft is obviously difficult. Contributing to existing aircraft is useful, but it doesn't teach you all of the skills required to create an aircraft from scratch. So people who want to do a full aircraft, need to start with something simple and "fun".
So what new contributors often do, is creating "fun" vehicles to learn how FlightGear works.
Aircraft like the ufo, ogel, bluebird etc were usually created by people in their initial phase of becoming familiar with FG - for example, the ogeL was made by the same guy who ended up modeling the Seneca, one of our most-developed (and maintained!) GA aircraft in FlightGear:
http://wiki.flightgear.org/SenecaAs you can tell from looking at the wiki, the ogeL aircraft developer has not only become the developer and maintainer of the Seneca aircraft in FlightGear, but he also has become a FlightGear core developer writing C++ code and maintaining the autopilot system for example, and he also happens to be a CPL-IFR/ME rated pilot in real life, who owns the Seneca he modeled for FlightGear.
Obviously, contributing to all these areas in FG takes time, a steep learning curve is an unfortunate but important part of it - rarely, do we see people who immediately start out with complex and well-developed aircraft, it's a process - and we all needed to get started at some point. Just because we contribute "playful" things to FG, doesn't necessarily mean that we are not interested in "realistic aviation" as you can see.
You will notice that many long-term contributors actually have an aviation-related professional background, be it flying (commercial, private, ATPL), software development, maths, physics, GIS or whatever. Among the long-term contributors are people with PhDs, Masters, commercial pilot rating, PPL or retired military pilots, and even one test pilot...
Still, 3D modeling is a completely new skills, and needs to be learned from scratch - which applies
especially to people who "only" have a real background in aviation, but not in computers and/or games.
Creating the ogeL was obviously simple and fun, creating aircraft like the Seneca requires tons of skills, experience and expertise that you don't end up having "automatically". And it requires tons of dedication and discipline.
I hope that puts things a little into perspective.
maybe I will learn C++ programming then I can contribute to FlightGear
It's a long-standing misconception that you need to know C++ in order to contribute to FlightGear, we have tons of non-coding areas actually, please see:
http://wiki.flightgear.org/VolunteerAnd even if you want to write code, there are tons of ways to write code, without writing C++ - such as using Nasal or GLSL instead.
Finally, keep in mind that most of us started contributing to non-coding areas in FG, even if we did have a professional software development background. In fact, we have people who do software development for a living, but still prefer doing 3D modeling in their spare time (for *fun*).
Not knowing C++ is a pathetic excuse not to contribute to FlightGear, if you are serious about contributing there are really MANY options:
http://wiki.flightgear.org/Volunteer